So the problem seems to be that too few emails will leave users stranded, and too many emails will make users annoyed. As a solution, I think an approach simila

So the problem seems to be that too few emails will leave users stranded, and too many emails will make users annoyed. As a solution, I think an approach similar to Beeminder might be good. They use a logarithmic distribution of emails. They send you an email every day at first (for the first 2 days), and if you don't respond, they dial it back to once a week. If you don't respond, they move it back to once a month or so, etc. I think the model is a good balance.